1. Field of the Invention
This invention relates to a spindle motor of the type in which a rotor or hub for supporting data carrier discs is rotatably supported on a central shaft by means of a pair of bearings. It also relates to a method of making or assembling such a motor.
2. Description of the Prior Art
As is well known, spindle motors are often used to drive magnetic data storage discs to write in or read out data by the aid of a magnetic read-write head. Obviously, such a motor is required to rotate as precisely controlled because otherwise the magnetic head may not be brought to an appropriate position relative to each disc for writing or reading. However, since the spindle motor is an assembly of various small parts, assembling or mounting errors will sometimes result, thereby failing to provide accurately controlled rotation. Such a problem becomes particularly serious in the case of mounting errors with respect to bearings.
To more clearly explain the problem described above, reference is now made to FIGS. 7 and 8 of the accompanying drawings which show a typical arrangement of a prior art spindle motor.
As shown in FIG. 7, the prior art spindle motor comprises a central shaft 100 fixed to a central boss portion 101a of a mount bracket 101. A hub or rotor 102 for drivingly supporting one or more magnetic discs (not shown) is rotatably supported on the central shaft by means of a pair of axially spaced ball bearings 103, 104 which are press-fitted in a central bore 102a of the hub. The central bore 102a is substantially closed by a closure ring 105. A stator 106 fixed on the bracket boss portion 1O1a cooperates with a plurality of rotor magnets 107 mounted on the hub 102 to rotate the hub relative to the shaft.
For assembly, the bearings 103, 104 are separately inserted into the central bore 102a of the hub 102 from above, as illustrated in FIG. 8. Thereafter, the shaft 100 is inserted through the bearings to be fitted into the boss portion 1O1a of the mount bracket 100 (FIG. 7), and the remaining parts are mounted in place.
As can be easily appreciated, the most important step of the assembling operation is the press-fitting of the bearings 103, 104 because improper mounting of these parts directly results in rotational deflection or vibration of the hub 102. In fact, however, one or both of the bearings are sometimes press-fitted improperly, as indicated in phantom lines in FIG. 8. This is due to the fact that the individual bearings, which themselves are very small parts, have a small axial length (thickness).
Generally, it is easy to properly press-fit a long cylinder for example into a corresponding bore because the cylinder, even if improperly oriented (e.g. the axis of the cylinder forming a slight angle relative to that of the bore) at the initial stage of insertion, will be corrected in orientation upon subsequent insertion. Therefore, mounting errors are unlikely to occur in press-fitting such a long body. However, the situation is quite different when inserting a thin coin coaxially into a corresponding bore, in which case the coin is very likely to be press-fitted in an inclined condition.
The bearing for the spindle motor is very thin (small in axial length), so that it may be roughly regarded as similar to the coin. For this reason, the bearing is sometimes mounted in an inclined condition, as shown in FIG. 8.
When one bearing is improperly press-fitted, it is not coaxial with the other bearing, consequently resulting in rotational deflection of the hub which causes writing or reading errors. Further, such improper mounting of the bearing also hinders smooth rotation of the hub in addition to damaging some rotational parts. Although it is possible to subsequently conduct positional correction of the improperly mounted bearing, such an operation is time-taking but yet fails to provide intended correction due to previous damaging or deformation of the bearing or the bore receiving it.